Medras is a way for an object to become tamai without it actually touching anything, it becomes tamai just by sitting, standing, or leaning.

A child’s walker is considered a seat and can become tamai through medras. It is considered a utensil and can be moved on Shabbos as it is not muktza. However, it cannot be walked with on Shabbos as it might come to make holes in the dirt and be considered choresh, plowing. R’ Yehuda permits a child’s walker to be walked with as it just presses and does not make holes.

Beitzah 3:1

You cannot catch fish from a fish pond on Yom Tov and you cannot give them food on Yom Tov. You can catch a non-domesticated animal or a bird from a closed area on Yom Tov. You can give food to them on Yom Tov. R’ Gamlial says not all closed areas are equal. Wherever it is missing getting caught catching is prohibited and if not missing get caught you can capture it on Yom Tov.